Breath-holding, the descended larynx, the diving
reflex, and the AAT/H

A common, ongoing claim by most
AAT/H proponents is that only humans and aquatic animals can hold their breath, that
the human descended larynx (compared to other mammals) is an aquatic trait,
and that only humans and aquatic animals exhibit the diving reflex.
This, they say, is evidence that we evolved in a situation with a lot of
diving going on, ie. aquatic.
However, even minimal study of the
claim by AAT/H proponents would have revealed to them that the diving reflex is actually
a universal trait found in all vertebrates, and that non-human,
non-aquatic animals can and do hold their breath.
I wouldn't hold
your breath waiting for AAT/H proponents to stop making this claim, though, cause
for years it's been pointed out to them and they haven't stopped yet.

Breath-holding in non-human animals

So contrary to the AAT/H claim,
humans are not the only non-aquatic mammal which can hold its breath. Various
monkeys, for instance, can and do hold their breath, and so do dogs.
(Another common and related AAT/H claim is that non-aquatic animals have no control
over their vocalizations, which should also surprise any dog owner.)

Lin (1982) reported on bradycardia
studies with dogs, using dogs which showed an ability to hold their breath.
("Breath-hold Diving in Terrestrial Mammals" by Yu-Chong Lin (Department
of Physiology, John A. Burns School of Medicine, University of Hawaii,
Honolulu, Hawaii).
In Exercise and Sport Sciences Review,
1982, vol. 10, pp. 270-307.)

By the way, seals and whales
don't hold their breath when they dive; they store oxygen in their blood,
and actually expel the air from their lungs as they dive.
This system --
radically different from humans -- is used by pinnipeds and cetaceans.

The Descended Larynx and the AAT/H

Contrary to the claims of AAT/H
proponents, a partially descended larynx -- in it's initial evolutionary
stages -- would be helpful when forming ever more complex sounds for communication,
just as Darwin showed a rudimentary eye would be a helpful adaptation which
could lead to a more complex "modern-style" eye.
The descended larynx of
humans is not fully descended at birth; instead it gradually descends during
the first four years of the infant's life.

The descended larynx of humans
therefore:
A) undergoes this change during
precisely the period when infants learn to speak -- in an ever more complex
manner; and
B) isn't fully formed for
the first four years of the infant's life.

If it were an adaptation
which "aquatic" hominids had developed from living in their environment,
these "aquatic" infants would without its supposed benefits for the first
four years of their lives.
And of course, since one of the unfortunate consequences of this feature is that we can get water into our lungs more easily, it's difficult to see why a feature which promotes drowning would be considered "aquatic".

The idea that this is an aquatic feature also doesn't explain why, in human males, the larynx descends even further at puberty (that's why male voices get dramatically lower at that time).
A far more sensible reason, based on research rather than speculation, will show up in a few paragraphs.

Even some AAT/H proponents, such as Marc Verhaegen,
have realized that this feature is not at all like the larynx of the vast majority of aquatic
mammals, although he still has described it as an "aquatic" trait.
When Elaine Morgan acknowledged Verhaegen's finding this out, she also admitted she just hadn't looked at the evidence on this -- excuse me? didn't look at the evidence? (that isn't someone I'd want to buy a used theory from...)

Morgan has stated that the descended larynx indicates habitual mouth breathing, which she says, without evidence, would be better for diving, but we see that other terrestrial mammals mouth breath just as humans do when exerting themselves -- common mammals like dogs and cats, so that doesn't add up.
Other than that, it's merely suggested that it must be an aquatic trait because it's seen only in humans and (a very few) aquatic mammals.
In short, there is no good explanation given why this would be an aquatic trait, and it turns out that there's excellent reasons to see that it isn't, for instance, the fact that, contrary to common AAT/H claims, there are other terrestrial mammals with descended larynxes.

Previously on this page I had mentioned a problem concerning the descended larynx as
a feature of our transitional population (as Morgan claims it) -- that the skulls of hominids
after the transition don't show the basicranial angle we see in humans that indicates a fully descended larynx.
It occurred several million years later.
However, it's possible that a descended larynx wouldn't leave this mark in the basicranial angle, but even this does nothing to comfort the AAT/H proponent who uses the larynx as an example of an aquatic trait, because it turns out that there are several terrestrial mammals (and even some birds) with permanently descended larynxes, and even more with larynxes that habitually descend when vocalizing.

For instance, chimpanzees have been found to have partially descended larynxes -- that's right, our closest relatives are similar to us in this feature, although their hyoid (the U-shaped bone that supports the tongue muscles) doesn't also descend as ours does.
(This gives us more fine control of vocalization than they have.)

Nishimura, Mikami, Suzuki, and Matsuzawa explain in the abstract of their 2003 paper ("Descent of the larynx in chimpanzee infants"):

We used magnetic resonance imaging to study the
development of three living chimpanzees and found that their
larynges also descend during infancy, as in human infants.
This
descent was completed primarily through the rapid descent of the
laryngeal skeleton relative to the hyoid, but it was not accompanied
by the descent of the hyoid itself.

But red deer and their North American relatives the wapiti, as well as fallow deer, have larynxes that are at least as descended as humans on a permanent basis, and more so when vocalizing.
It turns out that a lot of mammals' larynxes descend a lot when vocalizing, which shows us how such a feature could arise.
Others whose larynxes descend while vocalizing include koalas, "lions, tigers, and other members of the genus Panthera" (Hauser and Fitch 2003), dogs, pigs, goats and monkeys (Fitch 2002), and even at least some birds, like roosters and cardinals (Riede and Suthers, see refs).
It seems likely that as more animals are tested, we'll find still others with this feature.
W. Tecumseh Fitch, perhaps the leading researcher on this subject, explains why this wasn't seen decades back when Sir Victor Negus did his research:

Comparative Vocal Production and the Evolution of Speech:
Reinterpreting the Descent of the Larynx (2002)
W. Tecumseh Fitch:

9th pg.: The work on animal vocal anatomy dates mostly from the 19th century, and was
based exclusively upon dissections of dead animals.
By the time new techniques allowing
anatomical visualization of living animals were developed, such as x-ray film
(cineradiography) or MRI, the study of comparative anatomy had fallen from favor.
Although occasional critics pointed out that the crucial issue is not the resting position of the larynx, but
its position during vocalization (Nottebohm 1976), it is only recently that modern imaging
techniques have been applied to live vocalizing mammals (Fitch 2000a).
These data indicate
that mammals lower the larynx as a matter of course during vocalization, in some cases
approaching the "two-tube" configuration typical of adult humans.
Furthermore, new
anatomical analyses show that humans are not unique in our laryngeal position, because several
other species also have a permanently descended larynx.
Comparative Vocal Production and the Evolution of Speech:
Reinterpreting the Descent of the Larynx
W. Tecumseh Fitch

more from the same source:

9th pg.: These data indicate that the vocal tract
configuration of vocalizing animals, at least in dogs, pigs, goats and monkeys, is more similar to
that of human talkers than was previously inferred on the basis of dissections of dead animals.
In particular, all four nonhuman species examined can and do lower their larynges into the oral
cavity during loud vocalizations, either to a relatively minor degree (goats) or to a surprisingly
extensive degree (dogs).
W. Tecumseh Fitch

still more from the same source:

11th pg.: Finally, these data suggest that a lowered
larynx during vocalization is in fact a primitive trait that we share with other mammals,
rather than a uniquely human adaptation.
What is unusual about our species is that the adult
human larynx is permanently lowered, rather than dropping only during vocalization.
However, other recent data show that even this difference is not uniquely human.
W. Tecumseh Fitch

and a bit more from the same source:

11th pg.: But despite the
claims of Negus (1949), this position is not unique to humans.
In at least two species of deer, red
deer Cervus elaphus and fallow deer Dama dama, the larynx of postpubertal males is
permanently lowered to a resting position comparable to that in humans.
W. Tecumseh Fitch

Why does this feature arise?
Fitch has done a lot of work on this subject and has a lot of what seems to me to be excellent data strongly suggesting it's because it allows the animal to exaggerate its size when vocalizing.
This can be helpful in a variety of situations; for instance as a mating display (as especially seen, or rather heard, in the deer species he's studied) or other "status" displays, or quite possibly in scaring off predators.
The lowered larynx makes for a larger volume in the vocal apparatus and thus the tone is lowered -- just as a larger drum sounds deeper or a woodwind or brass instrument gives lower sounds when the internal volume is changed by moving keys or valves.
The research of Fitch and others shows that animals do make judgments about body size based on these audio cues.

How about the diving reflex: are humans well
adapted for diving?

Yes, we know humans can
swim and dive, but so can other non-aquatic animals.
Do humans show
the adaptations that are found in animals with a lifestyle in which diving
is important, as the AAT/H says it was for us?

AAT/H proponents say they do,
and that other terrestrial mammals don't show such responses as breath-holding
and the "diving" reflex.
The following scientists' writings show that the
facts say otherwise.
Note also that although Hardy said in 1960 that
the diving reflex was only found in humans and diving animals, contrary
evidence had been available for over 20 years before that.

1982 "Breath-hold Diving in
Terrestrial Mammals" by Yu-Chong Lin (Department of Physiology, John A.
Burns School of Medicine, University of Hawaii, Honolulu, Hawaii). In Exercise
and Sport Sciences Review, vol. 10, pp. 270-307.

pg. 271: However,
universality of the "diving" responses is well recognized in that these
responses exist in all vertebrates studied so far, differing only in intensity
and tempo.

pg. 272: Many previous
reports indicated that there is a slow development of bradycardia response
in humans.
Undoubtedly, these reports refer not to the onset but to the
slow attainment of the maximum bradycardia response.
It should be noted
that diving mammals arrive at maximum bradycardia much faster.

pg. 277: In contrast
to diving species, hypertension is a consistent finding in man during BH.

1983 Diving and Asphyxia:
A comparative study of animals and man by Robert Elsner (Professor
of Marine Science, University of Alaska) and Brett Gooden (Adelaide, South
Australia. Formerly Lecturer in Physiology, University of Nottingham).
Cambridge University Press: Cambridge.

pg. 66: One of the
most intriguing findings with regard to diving medicine is the occurrence
of cardiac arrhythmias during breath-hold diving.
This is in contrast to diving
animals who display a marked bradycardia but never show any cardiac arrhythmias
during diving.
In humans, it is common to find various types of arrhythmias
even during simply breath-holding.

1992 Diving and Subaquatic
Medicine, Third Edition. by Carl Edmonds, Christopher Lowry, and John
Pennefather.

pg. 357: In humans,
the diving reflex is more rudimentary and undeveloped.
Although there is
a diving bradycardia, it is often complicated by the development of idioventricular
foci producing ectopic beats.
ECG abnormalities are frequent during or
after the dive. T-wave inversion, premature ventricular excitation and
atrial fibrillation, together with other irregularities and dysrhythmias,
are common.

pg. 358: The drop
in cardiac output noted in the diving mammals is seen to only a slight
degree in humans.

pg. 33: While diving
for food, sea otters must conserve oxygen and contend with underwater pressure.
They generally forage in shallow waters of less than 60 feet, and their
feeding dives usually last one to two minutes.
Yet sea otters have been
known to dive to depths of 330 feet and remain under water for up to four
or five minutes.
In contrast, a person without oxygen passes out within
three minutes.
Sea otters can hold their breath for such a long time in
part because their lungs are nearly two-and-a-half times larger than the
lungs of a similar-sized mammal.
Large lungs help store oxygen and regulate
buoyancy while the otter floats on the surface.
And sea otters have flexible
ribs that allow the lungs to collapse under pressure.
Otters also have
cartilaginous airways connected directly to tiny, air-filled lung sacs
which help provide an unrestricted flow of oxygen to the blood.
In addition,
the sea otter's blood has a higher buffering capacity than that of non-diving
mammals, which helps the otter handle the excess carbon dioxide that accumulates
under pressure during a dive.

1985 The Natural History
of Sea Otters by Paul Chanin. Croom Helm: London and Sydney.

pp. 21-22: Like seals,
sea otters have a higher concentration of haemoglobin than terrestrial
mammals (and river otters) which enables them to carry more oxygen in their
blood.
They also have relatively large lungs which, compared to other marine
mammals, are twice as large as expected and are the reason for the sea
otter's large rib cage.
The lungs may form an important oxygen store but
it has been suggested that their importance is in providing buoyancy rather
than an air supply.
This is very necessary in an animal which has little
or no fat to give buoyancy and which often carries large stones with it
when feeding and foraging (see Chapter 3).

1990 The Pinnipeds: Seals,
Sea Lions, and Walruses by Marianne Riedmann. University of California
Press: Berkeley, L.A., and Oxford.

pp. 28-29: Bradycardia
and the channeling of blood to critical organs save oxygen.
Since muscles
are deprived of their usual supply of blood and oxygen, they compensate
by containing large amounts of a special compound called myoglobin, an
iron-bonding pigment related to hemoglobin that stores oxygen.
Myoglobin
helps the seal to tolerate the large accumulation of carbon dioxide in
its bloodstream that prompts breathing during periods of prolonged breath-holding.
Myoglobin therefore helps the animal to conserve oxygen during high-speed
movement or deep dives.
In fact, cetacean and pinniped muscles appear nearly
black when they are exposed to the air.
This dark coloration is due to
the extremely dense concentrations of deep red myoglobin in the muscle
tissues.

A seal's muscles are
also able to handle the high amounts of lactic acid that accumulates in
its system during periods of heavy or prolonged activity (such as diving),
which causes muscle exhaustion in land mammals.
The muscles of marine mammals
can function with insufficient oxygen for many hours.
Most of the time,
the muscles function aerobically (with oxygen), even though the seal may
be diving and exercising heavily.
They function anaerobically when oxygen
supplies are depleted by prolonged submergence.
When a seal surfaces from
a long dive, it takes several rapid and deep breaths until the body's oxygen
supply is replenished; at the same time, at least in harbor seals, a pronounced
tachycardia (extremely rapid heartbeat) occurs (Fedak et al. 1985).
Seals and whales even have an automatic mechanism that cuts off breathing if
they are knocked unconscious, so that their lungs will not fill with water.

In pinnipeds, the
oxygen-transporting circulatory system is very large.
A seal total blood
volume in relation to its body weight is 1.5-2 times greater than that
of other mammals.
According to Scheffer (1976), "In a full-grown walrus
the forked veins which drain the lower body are so enormous that a man
can draw them over his legs like a pair of pants!" Wickham et al. (1985)
and others (e.g., Lenfant et al. 1970) have identified a number of characteristics
of phocid seal blood that appear to help the seal to cope with the hypoxia,
or oxygen deficiency, of long dives.
Phocid blood has a greater capacity
than the blood of terrestrial mammals to store oxygen because phocids have
fewer and larger red blood cells with a higher concentration of hemoglobin,
which stores and carries oxygen.
And despite the high viscosity, or thickness,
of phocid blood, the rate of blood flow is high.

[My note: Since "Myoglobin helps
the seal to tolerate the large accumulation of carbon dioxide in its bloodstream
that prompts breathing during periods of prolonged breath-holding" why
wouldn't people have this handy feature, which would keep them from breathing
(and drowning) when they know perfectly well they shouldn't, ie. when under
water for too long.]

1990 The Natural History
of Seals by W. Nigel Bonner. Facts on File: New York, Oxford, and Sydney.

pp. 33-34: Breath-hold
capacity can be increased by taking more oxygen down with each dive. A
human diver will breathe deeply -- hyperventilate -- before diving, and
descend with lungs full of air.
There are disadvantages in this.
For a
start, full lungs increase buoyancy and so make it more difficult to descend;
secondly, there are pressure problems associated with diving with full
lungs.
Seals hyperventilate before diving, but they expel most of the air
from their lungs before they go down, and hence they do not depend on air
from the lungs for their oxygen when they dive.

Besides being stored
as a gas in the lungs, oxygen can be carried in physical solution in the
blood and tissue fluids, or chemically bound to haemoglobin in the red
blood cells or to another respiratory pigment, myoglobin, in the muscles.
In seals the major store is carried in the blood.
Seals have greater blood
volumes than terrestrial mammals.
A Weddell seal, for example, has about
150 ml of blood per kilogram bodyweight, about twice the value for Man.
Furthermore, seal blood contains more haemoglobin than human blood -- about
1.6 times as much.
The combined result of this is that the blood oxygen
stores per unit of body weight is about three or three and a half times
that of Man.
Gerry Kooyman (1981) has pointed out that because blubber
is more or less inert and represents about 30 percent of the seal's body
weight, a more realistic comparison might be to lean body weight, in which
case a seal's blood oxygen store would be about 5.3 times that of Man (though
it should be borne in mind that an obese human may have about the same
proportion of fat -- though differently distributed -- as a seal).
Not only does the seal have more haemoglobin in its blood than Man, it also
has very much more myoglobin in its muscles.
This protein, which resembles
haemoglobin in its ability to combine loosely with oxygen, is what gives
muscles their red color. [my notes here unfortunately left out a
partial sentence. sorry; the following statement refers to the muscles
of seals] Weddell, containing about ten times the myoglobin concentration
of human muscle, is almost black.